This application pertains to the art of electronic speed controls and more particularly to feedback speed controls. The invention finds particular application in controlling the speed of electric wheelchairs and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications, such as controlling the speed of other motorized patient care devices, electric vehicles, machine tool motors, and the like.
Conventionally, electric wheelchairs are driven by a pair of permanent magnet, DC electric motors connected with their driving wheels. A battery pack is mounted in the wheelchair for supplying electric power to the motors. A joystick, which is mounted with easy manual access to the operator, controls the amount and polarity of the power which is supplied from the battery to each of the drive motors. To move and accelerate the wheelchair, the control circuit supplies power to the motors. To slow the wheelchair, the control circuit stops supplying power to the motors. When a permanent magnet DC motor coasts, it functions as a generator. Commonly, a resistor is connected across the coasting motors to dissipate the generated electrical energy and slow the motors. This braking effect, which is commonly denoted as dynamic braking, is increased as the size of the braking resistance is decreased.
An exemplary prior art wheelchair speed control is illustrated in U.S. Pat. No. 4,157,123, issued June 5, 1979 to Keith S. Rodaway. In the Rodaway patent, a manually operated joystick is surrounded by four on-off switches for independently applying power from the battery to the right and/or left wheel motors with either a forward or reverse polarity. The joystick can be tipped to actuate one of the switches for the right motor and/or one for the left motor. By operating the appropriate switches, the joystick can cause the chair to move forward, rearward, turn to the right, turn to the left, or pivot. When the joystick is not supplying power of either polarity to one or both of the motors, a power dissipating resistor is connected across that motor. To select the speed, a fast/slow switch is provided separate from the joystick.
One of the problems with the Rodaway circuit is that the speed of the wheelchair is affected by several uncontrolled factors. When the chair is proceeding downgrade, it will move faster and, when it is moving upgrade, it will move slower. Further, operating on-off switches rather than continuously variable switches, such as a potentiometer, with the joystick limits the selection of speed, handling, and performance. For example, there is a single turning radius for right and lefthand turns which, like the speed, varies if the chair is on a grade. Another problem is that the wheelchair can gain excessive speed on a downgrade, because the power to the motors and the connection of the braking resistor across the motors is controlled by the joystick position and not the actual speed.
Another exemplary prior art wheelchair control circuit is illustrated in U.S. Pat. No. 4,059,786, issued Nov. 22, 1977 to Michael Lloyd Jones et al. In the Jones patent, the speed of the wheelchair is controlled by a joystick which operates a potentiometer providing a continuously variable analog voltage indicative of the selected speed. A pulse width modulator converts the modulus of the potentiometer voltage into a proportionate amount of power for the driving motor. A reversing relay is provided for changing the polarity with which the power is provided to the motor for forward and reverse directions. Like the later Rodaway patent, the Jones et al. patent includes a dynamic braking resistor which is shunted across the motor to slow the wheelchair. The Jones circuit further has an armature voltage detector for enabling the forward/reverse relay to change states only when there is substantially no voltage across the armature, i.e., when the motor is stationary. An overload detector including a resistor in series with the power supply and the motor prevents an excessive amount of power from being supplied to the motor regardless of the position of the joystick.
One problem with the Jones control is that, like the Rodaway control, the amount of driving power supplied to each motor depends only on the joystick position. No accommodation is made for the actual speed of the chair. On uneven terrain, the speed and handling of the chair can become unpredictable and hard to control.
The present invention contemplates a new and improved speed control circuit which overcomes the above-referenced problems and others. It provides a speed control circuit which accurately holds the selected speed of a wheelchair even over uneven terrain.